Add backend selection to viewer's GUI

BUG=skia:

Change-Id: I3314013538792c2aa82cc49f3f072aab2cdc4a55
Reviewed-on: https://skia-review.googlesource.com/9079
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
2 files changed